Gate

Put each stack in its own resource groups and scope the runner’s role to them. Azure Policy can deny a write whose plunge-stack tag names another stack:

{
  "if": {
    "allOf": [
      { "field": "tags['plunge-stack']", "exists": "true" },
      { "field": "tags['plunge-stack']", "notEquals": "myproject/prod" }
    ]
  },
  "then": { "effect": "deny" }
}